Modulation of D1 protein turnover under cadmium and heat stresses monitored by [S-35]methionine incorporation

This study examined the response of Photosystem II (PSII) to heavy metal an d heat stresses and their combination in two higher plant species, pea (Pis um sativum) and broad beans (Vicia faba). The photochemical activity of PSI I was monitored as the maximum fluorescence yield of PSII, Fv/Fm, in leaves and as DCPIP photoreduction in thylakoids isolated from control and treate d leaves. The pre-treatment of plants with cadmium (1 mM CdCl2) induced a t emporary protection of PSII activity against heat stress (38 degrees C). On the contrary, pre-treatment at the high temperature did not protect PS II against heavy metal (1 mM CdCl2) stress. The pulse-chase experiments with [ S-35]methionine demonstrated that the two types of stress had different con sequences on D1 protein turnover. When the plants were subjected to the cad mium treatment (Cd-treatment), net D1 synthesis was first stimulated, and t hen inhibited. Heat stress, however, caused an immediate decline of net D1 synthesis. In response to the simultaneous imposition of both stresses, D1 turnover remained stable initially for several hours, but was greatly inhib ited under prolonged treatment. Both treatments resulted in a decrease of t he D1 level. The symptoms of heavy metal stress were particularly evident w hen D1 turnover was scarcely stimulated. (C) 1999 Published by Elsevier Sci ence Ireland Ltd. All rights reserved.